1. Field of the Invention
The present invention generally relates to an amplifier, and particularly relates to an amplifier which can substantially avoid a duty-ratio error.
2. Description of the Related Art
As technology makes further progress in size reduction and lowering of power consumption of communication equipment, there is an increasing demand for circuits used in communication equipment to operate with low voltages. An IF (intermediate frequency) amplifier unit, which is generally provided at a stage prior to an FM demodulation unit, is required to provide a sufficient gain and a small waveform distortion under low-voltage conditions.
FIG. 1 is a circuit diagram of a related-art IF amplifier unit.
The IF amplifier unit of FIG. 1 includes amplifiers A1 and A2, a comparator C1, and resistors R1 through R5.
The amplifier A1 receives an input signal via resistor R1 at one input node when the input signal is supplied to an input terminal, and receives a reference voltage VSG at the other input node. The amplifier A1 amplifies the input signal with reference to the reference voltage VSG. The input node of the amplifier A1 that is connected to the resistor R1 is connected to an output node of the amplifier A1 via the resistor R2.
The signal amplified by the amplifier A1 takes two different routes to be input to the amplifier A2, one being through the resistor R3 and the other being through the resistor R4. The amplifier A2 serves to cancel an offset of the input signal relative to the reference voltage VSG. The input node of the amplifier A2 that is connected to the resistor R3 is connected to the output node of the amplifier A2 via the resistor R5. The other input node of the amplifier A2 that is connected to the resistor R4 is connected to the ground via a capacitor.
A signal output from the amplifier A2 is supplied to the comparator C1. The comparator C1 receives the reference voltage VSG in addition to the signal from the amplifier A2, and compares the signal with the reference voltage VSG to output a result of the comparison at its output node.
The IF amplifier unit of FIG. 1 exhibits problems when the input signal supplied to the input terminal exceeds a certain threshold voltage level.
FIGS. 2A through 2C are illustrative drawings of waveforms for explaining problems that arise when an input signal exceeds a threshold voltage level.
FIG. 2A shows an input signal together with a power voltage level and a ground voltage level where the input signal is supplied to the input terminal. As long as the input signal stays below a threshold voltage level, no problem is observed.
When the input signal exceeds the threshold voltage level, however, a signal output from the amplifier A1 behaves as shown in FIG. 2B. As shown in FIG. 2B, the output signal of the amplifier A1 has an average voltage level V2 that is different from the reference voltage VSG. This is because clamping occurs when the input signal to the input terminal exceeds the threshold voltage level. When the signal as shown in FIG. 2B is supplied to the amplifier A2, an output signal of the amplifier A2 behaves as shown in FIG. 2C.
Operating with reference to the reference voltage VSG, the comparator C1 produces an output signal that suffers a phase displacement amounting to a temporal displacement dT in comparison with an output signal that would be obtained when a difference between the average voltage level V2 and the reference voltage VSG is insignificant. This phase displacement leads to an error in a duty ratio.
Accordingly, there is a need for an IF amplifier which can reduce a difference between a reference voltage and an average voltage so as to suppress an error of a duty ratio.